Technical Field
This disclosure pertains to a method for converting organic salts produced in buffered mixed-acid fermentation into acids, followed by extraction to recover the acids from an aqueous solution. Embodiments of the disclosure also pertain to integrating such aspects with the conversion of the acids to products, such as ketones and acetates.
Background
Typical conversion of organic salts to acid requires the addition of acid or the regeneration of resins using acids, which results in a large waste stream of salts and inability to regenerate the base to be used in the fermentation. This is costly and not environmentally friendly.
Terminology and Glossary
Throughout this specification, the following terminology applies:
“VFAs”—Abbreviation for “volatile fatty acids”, which are the organic acids of carboxylic type produced in anaerobic fermentation by naturally occurring consortia of anaerobic bacteria. Namely these VFAs are short- and medium-chain fatty acids such as acetic, propionic, butyric, iso-butyric, valeric, iso-valeric, caproic, enanthic, caprylic, pelargonic acids, and combinations thereof. Smaller amount of higher acids, such as decanoic and undecanoic have also been detected in analyses. These acids are neutralized in the fermentation to control pH thus ending up with the salts of the acids also known as VFA salts. The terms VFA (or VFAs in plural), short- and medium-chain fatty acids, carboxylic acids, or organic acids may be used interchangeably.
“EAU”—Electrochemical Acidification Unit. Generic term that refers to any unit that employs techniques for electrochemically acidifying a solution.
“LMW Ketones”—Low-molecular-weight—used to denote ketones that are preferably, but not limited to C4 and C5 ketones.
“HMW Ketones”—High-molecular-weight—used to denote ketones that are preferably, but not limited to, C8 and C9 ketones.
“CED”—Abbreviation for “conventional electrodialysis”
“EDI”—Abbreviation for “electrodionization”
“EDBM”—Abbreviation for “Electrodialysis with bi-polar membranes”.
“OLAL”—Organic liquid-Aqueous liquid
“GOLAL”—Gas-Organic liquid-Aqueous liquid
“MCFA”—Medium-chain fatty acid
“Biomass”—Any biological material.
In addition to what is typically done to acidify fermentation broth, which is adding acids, which produces waste streams and has high operating expenditures, some processes also use the so called “acid springing process”. This entails contacting a carboxylate solution (e.g., ammonium) with a solvent, such as trioctylamine (TOA) or TOPO (Trioctylphosphine oxide) with the fermentation broth (made out of calcium or ammonium salts of the organic acids). Carbon dioxide could be added or the ammonia could be removed by evaporation. The resulting complex of TOA or TOPO would then be heated to decompose and release the acid.
Several problems exist with such processes, including the fact that the end product is seldom only calcium or only ammonium salts; instead, there is routinely a mixture of salts that may contain problematic components or impurities, such calcium, ammonium, sodium, potassium, magnesium, iron, etc. When there is a mixture of these cations, the process described above cannot be applied efficiently or effectively. In addition, the solvents used are very expensive, so losses are of serious concern.
Organic acids are the most common metabolites produced in fermentations. Most microorganisms produce organic acids in preference to other types of compounds such as alcohols. Such is the case because there is a thermodynamic advantage to producing organic acids as their energy state is lower than most other metabolites such as alcohols. This thermodynamic advantage makes their production a more robust process. It is, therefore, advantageous to allow microorganisms to produce organic acids.
In addition, as biochemicals, organic acids are valuable compounds. Citric acid, succinic acid, ascorbic acid, pyruvic acid, gluconic acid, lactic acid, formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid are some examples of valuable organic acids that can be produced by fermentation. However, recovery of such acids from the fermentation broth, especially those that are soluble in water, has been over the years considered a challenge.
Organic acids generally have a higher boiling point than water, which makes it difficult to separate them by distillation; therefore, typically extraction is the preferred method. However, acids tend to lower pH of the fermentation, which in general inhibits microorganism growth. In addition, most acids are toxic to microorganisms in their unionized state. As a result, a buffering agent (e.g., sodium hydroxide) is added to control pH. Such neutralization of the acids generates the salts of the acid, which due to their ionic state are difficult to recover by extraction.
Because the acid must be in its non-ionic state to be efficiently extracted, processes that produce organic acids typically must either operate the fermentation at a low pH, which is not always possible due to inhibitions, or they must add acids, such as sulfuric acid, to the resulting fermentation effluent. Although some salts of the mineral acid, such as sulfate salts, maybe decomposed back into the mineral acid (e.g., sulfate salts decompose into sulfur dioxide, SO2, which may be used as an acid itself or it may be converted into sulfur trioxide, SO3, which may be dissolved in water to recovery sulfuric acid), such conversion occurs at very high temperatures, so it may be costly; therefore, generally the addition of acid generates a waste stream of salts, such as sodium sulfate, which is undesirable as they constitute an environmental hazard and a significant operational cost for disposal.
Electrochemical Techniques
The best known electrochemical process for acidification is known as electrodialysis, particularly using bi-polar membranes (EDBM). Electrodialysis (ED) was discovered in 1890, with most of its breakthrough developments occurring in the 1930's, 1940's and 1950's. Since then, research, uses and industrial implementation of ED have increase exponentially.
FIGS. 1A-1C shows conventional 3- (FIG. 1A) and 2-chamber (FIGS. 1B-1C) EDBMs. The basic set up of an ED system makes use of a direct current supply, electrodes 100A and 100B, perm-selective ion-exchange membranes 101A & 101B, solvents 103, and electrolytes 104. The basic principle of its operation is that direct current is applied to electrodes 100A and 100B to allow the positive (cations 105) or negative (anions 106) electrolytes in the solvent to be transported towards the opposite charged electrode (100A for the cations and 100B for the anions), while the ion-exchange membranes 101A and 101B allow passage or retain the electrolytes 104 and thus achieve the desired effect. As mentioned, among the electrochemical acidification techniques, Electrodialysis with Bi-polar Membranes (EDBM) is probably the best known method, which make use of bi-polar membranes 102 (membranes with positive and negative charges) to split water and generate acid (hydrogen/hydronium ions) or base (hydroxide ions).
Other electrochemical techniques produce the hydrogen/hydronium ions at the electrode. A good example of these techniques was developed by Gilliam et al. with focus on the production of alkaline solutions. Such system, dubbed as Alkalinity Based on Low Energy (ABLE) is disclosed in U.S. Pat. Nos. 7,993,511, 7,993,500, 7,875,163, 7,790,012, U.S. patent application Ser. No. 12/989,781, U.S. patent application Ser. No. 13/021,355, U.S. patent application Ser. No. 12/952,665 and U.S. patent application Ser. No. 12/991,898 incorporated by reference in their entirety for all purposes. The ABLE technique oxidizes hydrogen at the anode to hydrogen/hydronium ions and produces hydroxide ions at the cathode. The acidic and basic solutions are separated by perm-selective membranes. Two variations of this technique have been devised and their description follows:
The first technique consumes electricity and produces hydrogen gas at the cathode, while releasing hydroxide ions into the solution in which the cathode is submerged. The hydrogen produced may be directed to the anode to be oxidized, which releases the hydrogen ions into the solution in which the anode is submerged thus acidifying it.
In the second technique, which is very similar to how a fuel cell operates, oxygen is supplied to the cathode so that it is reduced to produce hydroxide ions, which are released into the solution in which the cathode is submerged. At the anode, on the other hand, extraneous hydrogen gas is introduced and oxidized to produce hydrogen ions, which are released into the solution in which the anode is submerged. In this technique, electricity may be generated and exported rather than consumed.
In both of Gilliam's techniques appropriate and suitable catalysts may be applied in the anode and cathode to improve the efficiency of the reaction. Also in both of Gilliam's techniques, to make the reaction more favorable, carbon dioxide may be introduced into the solution in which the cathode is submerged to lower the pH and form carbonate and bicarbonate ions as the hydroxide ions are released.
Ion-Exchange Background
Ion exchange is a well established technique for recovery, purification, separation and decontamination of aqueous and other ion-containing solutions using a polymeric or mineral ‘ion exchanging’ media. Such media in its fresh or regenerated state carries a certain type of ion, be it a cation with positive charge or an anion with negative charge and it releases these cations or anions, while at the same time uptakes cations or anions, respectively from the ion-containing solution, thus causing ‘ion exchange’. The media will keep exchanging ions until it is exhausted of the original ion. At that point, the media needs to be regenerated by passing through a concentrated solution containing the original ion it held. Because of this regeneration step, typically, to allow uninterrupted continuous operation, two or more ion exchange beds are operated, so that one or more beds may operate, while others are being regenerated.
Known ion exchangers of the mineral type are zeolites and clay. However, more efficient systems employ polymeric resins, such as, but not limited to, those manufactured by Dow Water Solutions (Dowex™ and Amberlite™ resins). Within the group of polymeric resins, there are anion exchange resins and cation exchange resins.
One consideration in regard to use of ion exchange as compared to electrochemical techniques is that the capital cost is lower. However, another consideration is that the typical operation mode for ion exchange is that once the ion exchange bed needs to be regenerated, an acid, such as sulfuric or hydrochloric acid, has to be used thus generating a waste stream of inorganic salts that must be dealt with. Also, in addition to the operating costs for the acid and regeneration waste disposal, the base employed as buffering agent for pH control in the fermentation is not recovered and must be replenished, which adds even more to the operating costs. Such waste issues and non-recoverability of the buffering agent for the fermentation raises not only economic, but also environmental concerns, which have made researchers consider electrochemical techniques over ion exchange as the more feasible, economical and environmentally friendly option.
To ensure that a process using cation exchange may be both economically and environmentally sustainable, a different method for regenerating the cation exchange media is necessary. For such purpose, many have proposed the regeneration of the media using high-pressure carbon dioxide and water. Such regeneration produces the bicarbonate (HCO3−) salt of the cation absorbed (e.g, Na+, K+). Pressures that have been used for this process range from as low as about 15 psi to over 3600 psi. The CARIX process, for instance, is a well established process that has been used for water demineralization, which uses high-pressure CO2 for regeneration. After regeneration, when the pressure is released, a lot of the CO2 is released; therefore, CO2 recycle is sometimes implemented.
Liquid-Liquid Extraction Under High-Pressure Carbon Dioxide
Several researchers have proposed the recovery of carboxylic acids from their salts using high-pressure CO2 for acidification, while using liquid-liquid extraction to remove the acids from the aqueous phase. Pressures as high as 50 bars were tested, but no improvement was typically observed above 30 to 40 bars. Baniel et al. patented a process for extraction of lactic acid using amines as the extracting solvent under high pressure CO2.
The discussion above establishes that there is the need for effective and cost-efficient processes that are able to convert the organic salts into the non-ionized organic acid to allow efficient and cost-effective extraction without the production of undesirable streams (e.g., salt waste streams).